In semiconductor manufacturing devices and the like, for example, a wafer is arranged inside a reaction chamber held in a high vacuum state by a vacuum pump, a reaction gas is introduced, and a thin film is formed using CVD or the like. A work piece has to be transported inside the reaction chamber in a sealed state. In a transporting mechanism used to perform such a task, there has to be a complete, air-tight partition inside the reaction chamber between the arm portion actually holding the work piece and the drive mechanism for transmitting power from outside of the reaction chamber to the arm portion. Also, the generation of dust and the like has to be maximally suppressed on the reaction chamber side. As a result, a drive mechanism is desired for the arm portion inside the reaction chamber which does not generate abrasion powder, lubricant mist, and the like.
In such semiconductor manufacturing devices, a magnetic fluid sealing device such as the one shown in FIG. 10 is used. This magnetic fluid sealing device uses magnetic circuit forming means composed of a pair of pole pieces 102, 103 serving as magnetic pole pieces, and a magnet 104 serving as magnetic force generating means interposed between the pair of pole pieces 102, 103. The pair of pole pieces 102, 103 is installed in a housing 112 via O rings 105, 106 for improving the sealing properties; and a magnetic circuit is formed by the pole pieces 102, 103, the magnet 104, magnetic fluid 107, and a shaft 111 made of a magnetic material. The magnetic fluid 107 is held between the pole pieces 102, 103 and a plurality of ring-shaped protruding ends formed in the shaft 111, and a sealing function is provided for holding the vacuum side, which is the side to be sealed, in a vacuum state (referred to below as “Prior Art 1”).
A bearing 110 serving as a bearing section is arranged on the atmosphere side of such a magnetic fluid sealing device 101. The bearing 110 is typically arranged on the atmosphere side of the magnetic fluid sealing device 101, as the device is averse to dust generated by the bearing 110. An angular bearing or the like can be used as the bearing 110, and grease is often used as the lubricant for this bearing 110.
However, in Prior Art 1, the grease usually mixes with the thickeners in the base oil, and this causes some oil separation. This case becomes more pronounced at higher temperatures. When the bearing is of a single-supported-end type as shown in FIG. 10, the separated oil flows out of the bearing 110, mixes with the magnetic fluid 107, and causes the magnetic fluid 107 to deteriorate. A problem is presented in that the pressure resistance and vacuum properties are adversely affected, and the life of the magnetic fluid sealing device 101 is reduced (referred to below as the “Problem 1”).
Also, since the separated oil flows out from the bearing 110 on the atmosphere side and dries out, torque is increased. This may damage the bearing in the worst case. Further, when grease is added to the bearing, the device has to be disassembled. This imposes a cumbersome operation.
In a dual-supported-side-type magnetic fluid sealing device in which a bearing is arranged on the vacuum side, Problem 1 occurs as with a single-supported-side-type bearing. A further problem is presented in that bubbles and moisture are discharged into the vacuum, which degrades the vacuum quality inside the vacuum chamber, and pressure fluctuations to occur (referred to below as “Problem 2”).
In view of Problem 1 mentioned above, there is known a device in which an oil receiving portion curving downward on the housing side is provided on the upper surface of the pole piece on the atmosphere side. When the grease experiences some oil separation in the bearing and the separated oil flows out from the bearing, it is collected in the oil receiving portion in the bottom portion of the bearing to prevent the oil from admixing with the magnetic fluid (referred to below as “Prior Art 2;” e.g., refer to Patent Document 1).
Also, in view of Problem 2 mentioned above, there is known a device in which, as shown in FIG. 11, magnetic fluid is used instead of grease as a lubricant for first and second ball bearings 113, 114 rotatably supporting the rotary output shaft 121 in a rotary transmission device for transmitting power such as turning force and the like between a vacuum side and an atmosphere side partitioned in airtight fashion by a partitioning wall 120 (referred to below as “Prior Art 3;” e.g., refer to Patent Document 2). Prior Art 3 has an annular first spacer 115 interposed between the outer races of the first and second ball bearings 113, 114, an annular second spacer 116 interposed between the inner races, an annular stepped surface 122a, and a nut 117, whereby the position in the axial direction of the outer race and the inner race of the first and second ball bearings 113, 114 are determined. In order to constitute a magnetic circuit, the first spacer 115 is formed from a ferromagnet such as ferritic or martensitic stainless steel, the axial ends are magnetized to create an N pole and an S pole, and at least a shaft portion 122 of the rotary output shaft 121 is formed from a magnet. In addition, the ball bearings 113, 114 are also made of a commonly used metal magnetic material, the second spacer 116 is made of a non-magnetic material, and the periphery of the contact portions of the ball bearings 113, 114 is formed in a state of being covered by a magnetic fluid.